--- a/dom/base/BlobSet.h
+++ b/dom/base/BlobSet.h
@@ -27,18 +27,17 @@ public:
   nsresult AppendVoidPtr(const void* aData, uint32_t aLength);
   nsresult AppendString(const nsAString& aString, bool nativeEOL, JSContext* aCx);
   nsresult AppendBlobImpl(BlobImpl* aBlobImpl);
   nsresult AppendBlobImpls(const nsTArray<RefPtr<BlobImpl>>& aBlobImpls);
 
   nsTArray<RefPtr<BlobImpl>>& GetBlobImpls() { Flush(); return mBlobImpls; }
 
   already_AddRefed<Blob> GetBlobInternal(nsISupports* aParent,
-                                         const nsACString& aContentType,
-                                         ErrorResult& aRv);
+                                         const nsACString& aContentType);
 
 protected:
   bool ExpandBufferSize(uint64_t aSize)
   {
     using mozilla::CheckedUint32;
 
     if (mDataBufferLen >= mDataLen + aSize) {
       mDataLen += aSize;

--- a/dom/base/File.cpp
+++ b/dom/base/File.cpp
@@ -252,29 +252,26 @@ Blob::ToFile()
   } else {
     file = new File(mParent, mImpl);
   }
 
   return file.forget();
 }
 
 already_AddRefed<File>
-Blob::ToFile(const nsAString& aName, ErrorResult& aRv) const
+Blob::ToFile(const nsAString& aName) const
 {
   nsAutoTArray<RefPtr<BlobImpl>, 1> blobImpls;
   blobImpls.AppendElement(mImpl);
 
   nsAutoString contentType;
   mImpl->GetType(contentType);
 
   RefPtr<MultipartBlobImpl> impl =
-    MultipartBlobImpl::Create(blobImpls, aName, contentType, aRv);
-  if (NS_WARN_IF(aRv.Failed())) {
-    return nullptr;
-  }
+    new MultipartBlobImpl(blobImpls, aName, contentType);
 
   RefPtr<File> file = new File(mParent, impl);
   return file.forget();
 }
 
 already_AddRefed<Blob>
 Blob::CreateSlice(uint64_t aStart, uint64_t aLength,
                   const nsAString& aContentType,
@@ -345,21 +342,17 @@ Blob::WrapObject(JSContext* aCx, JS::Han
   return BlobBinding::Wrap(aCx, this, aGivenProto);
 }
 
 /* static */ already_AddRefed<Blob>
 Blob::Constructor(const GlobalObject& aGlobal, ErrorResult& aRv)
 {
   RefPtr<MultipartBlobImpl> impl = new MultipartBlobImpl();
 
-  impl->InitializeBlob(aRv);
-  if (NS_WARN_IF(aRv.Failed())) {
-    return nullptr;
-  }
-
+  impl->InitializeBlob();
   MOZ_ASSERT(!impl->IsFile());
 
   RefPtr<Blob> blob = Blob::Create(aGlobal.GetAsSupports(), impl);
   return blob.forget();
 }
 
 /* static */ already_AddRefed<Blob>
 Blob::Constructor(
@@ -853,17 +846,17 @@ BlobImplFile::GetMozFullPathInternal(nsA
 {
   NS_ASSERTION(mIsFile, "Should only be called on files");
   aRv = mFile->GetPath(aFilename);
 }
 
 uint64_t
 BlobImplFile::GetSize(ErrorResult& aRv)
 {
-  if (BlobImplBase::IsSizeUnknown()) {
+  if (IsSizeUnknown()) {
     NS_ASSERTION(mWholeFile,
                  "Should only use lazy size when using the whole file");
     int64_t fileSize;
     aRv = mFile->GetFileSize(&fileSize);
     if (NS_WARN_IF(aRv.Failed())) {
       return 0;
     }
 
@@ -904,17 +897,17 @@ BlobImplFile::GetType(nsAString& aType)
 
   aType = mContentType;
 }
 
 int64_t
 BlobImplFile::GetLastModified(ErrorResult& aRv)
 {
   NS_ASSERTION(mIsFile, "Should only be called on files");
-  if (BlobImplBase::IsDateUnknown()) {
+  if (IsDateUnknown()) {
     PRTime msecs;
     aRv = mFile->GetLastModifiedTime(&msecs);
     if (NS_WARN_IF(aRv.Failed())) {
       return 0;
     }
 
     mLastModificationDate = msecs;
   }
@@ -1123,29 +1116,21 @@ BlobImplTemporaryBlob::GetInternalStream
     new nsTemporaryFileInputStream(mFileDescOwner, mStartPos, mStartPos + mLength);
   stream.forget(aStream);
 }
 
 ////////////////////////////////////////////////////////////////////////////
 // BlobSet implementation
 
 already_AddRefed<Blob>
-BlobSet::GetBlobInternal(nsISupports* aParent,
-                         const nsACString& aContentType,
-                         ErrorResult& aRv)
+BlobSet::GetBlobInternal(nsISupports* aParent, const nsACString& aContentType)
 {
-  RefPtr<BlobImpl> blobImpl =
-    MultipartBlobImpl::Create(GetBlobImpls(),
-                              NS_ConvertASCIItoUTF16(aContentType),
-                              aRv);
-  if (NS_WARN_IF(aRv.Failed())) {
-    return nullptr;
-  }
-
-  RefPtr<Blob> blob = Blob::Create(aParent, blobImpl);
+  RefPtr<Blob> blob = Blob::Create(aParent,
+    new MultipartBlobImpl(GetBlobImpls(),
+                          NS_ConvertASCIItoUTF16(aContentType)));
   return blob.forget();
 }
 
 nsresult
 BlobSet::AppendVoidPtr(const void* aData, uint32_t aLength)
 {
   NS_ENSURE_ARG_POINTER(aData);
 

--- a/dom/base/File.h
+++ b/dom/base/File.h
@@ -113,18 +113,17 @@ public:
   // otherwise it returns a new File object with the same BlobImpl.
   already_AddRefed<File> ToFile();
 
   // XXXjwatt Consider having a ToDirectory() method. The need for a FileSystem
   // object complicates that though.
 
   // This method creates a new File object with the given name and the same
   // BlobImpl.
-  already_AddRefed<File> ToFile(const nsAString& aName,
-                                ErrorResult& aRv) const;
+  already_AddRefed<File> ToFile(const nsAString& aName) const;
 
   already_AddRefed<Blob>
   CreateSlice(uint64_t aStart, uint64_t aLength, const nsAString& aContentType,
               ErrorResult& aRv);
 
   void
   GetInternalStream(nsIInputStream** aStream, ErrorResult& aRv);
 
@@ -794,20 +793,16 @@ public:
                                       ErrorResult& aRv) const override;
   virtual void GetInternalStream(nsIInputStream** aInputStream,
                                  ErrorResult& aRv) override;
 
   void SetPath(const nsAString& aFullPath);
 
   virtual void LookupAndCacheIsDirectory() override;
 
-  // We always have size and date for this kind of blob.
-  virtual bool IsSizeUnknown() const { return false; }
-  virtual bool IsDateUnknown() const { return false; }
-
 protected:
   virtual ~BlobImplFile() {
     if (mFile && mIsTemporary) {
       NS_WARNING("In temporary ~BlobImplFile");
       // Ignore errors if any, not much we can do. Clean-up will be done by
       // https://mxr.mozilla.org/mozilla-central/source/xpcom/io/nsAnonymousTemporaryFile.cpp?rev=6c1c7e45c902#127
 #ifdef DEBUG
       nsresult rv =

--- a/dom/base/MultipartBlobImpl.cpp
+++ b/dom/base/MultipartBlobImpl.cpp
@@ -20,47 +20,16 @@
 #include "nsIXPConnect.h"
 #include <algorithm>
 
 using namespace mozilla;
 using namespace mozilla::dom;
 
 NS_IMPL_ISUPPORTS_INHERITED0(MultipartBlobImpl, BlobImpl)
 
-/* static */ already_AddRefed<MultipartBlobImpl>
-MultipartBlobImpl::Create(const nsTArray<RefPtr<BlobImpl>>& aBlobImpls,
-                          const nsAString& aName,
-                          const nsAString& aContentType,
-                          ErrorResult& aRv)
-{
-  RefPtr<MultipartBlobImpl> blobImpl =
-    new MultipartBlobImpl(aBlobImpls, aName, aContentType);
-  blobImpl->SetLengthAndModifiedDate(aRv);
-  if (NS_WARN_IF(aRv.Failed())) {
-    return nullptr;
-  }
-
-  return blobImpl.forget();
-}
-
-/* static */ already_AddRefed<MultipartBlobImpl>
-MultipartBlobImpl::Create(const nsTArray<RefPtr<BlobImpl>>& aBlobImpls,
-                          const nsAString& aContentType,
-                          ErrorResult& aRv)
-{
-  RefPtr<MultipartBlobImpl> blobImpl =
-    new MultipartBlobImpl(aBlobImpls, aContentType);
-  blobImpl->SetLengthAndModifiedDate(aRv);
-  if (NS_WARN_IF(aRv.Failed())) {
-    return nullptr;
-  }
-
-  return blobImpl.forget();
-}
-
 void
 MultipartBlobImpl::GetInternalStream(nsIInputStream** aStream,
                                      ErrorResult& aRv)
 {
   *aStream = nullptr;
 
   nsCOMPtr<nsIMultiplexInputStream> stream =
     do_CreateInstance("@mozilla.org/io/multiplex-input-stream;1");
@@ -151,29 +120,25 @@ MultipartBlobImpl::CreateSlice(uint64_t 
       blobImpls.AppendElement(lastBlobImpl);
     } else {
       blobImpls.AppendElement(blobImpl);
     }
     length -= std::min<uint64_t>(l, length);
   }
 
   // we can create our blob now
-  RefPtr<BlobImpl> impl = Create(blobImpls, aContentType, aRv);
-  if (NS_WARN_IF(aRv.Failed())) {
-    return nullptr;
-  }
-
+  RefPtr<BlobImpl> impl =
+    new MultipartBlobImpl(blobImpls, aContentType);
   return impl.forget();
 }
 
 void
-MultipartBlobImpl::InitializeBlob(ErrorResult& aRv)
+MultipartBlobImpl::InitializeBlob()
 {
-  SetLengthAndModifiedDate(aRv);
-  NS_WARN_IF(aRv.Failed());
+  SetLengthAndModifiedDate();
 }
 
 void
 MultipartBlobImpl::InitializeBlob(
        JSContext* aCx,
        const Sequence<OwningArrayBufferOrArrayBufferViewOrBlobOrString>& aData,
        const nsAString& aContentType,
        bool aNativeEOL,
@@ -217,51 +182,47 @@ MultipartBlobImpl::InitializeBlob(
 
     else {
       MOZ_CRASH("Impossible blob data type.");
     }
   }
 
 
   mBlobImpls = blobSet.GetBlobImpls();
-  SetLengthAndModifiedDate(aRv);
-  NS_WARN_IF(aRv.Failed());
+  SetLengthAndModifiedDate();
 }
 
 void
-MultipartBlobImpl::SetLengthAndModifiedDate(ErrorResult& aRv)
+MultipartBlobImpl::SetLengthAndModifiedDate()
 {
   MOZ_ASSERT(mLength == UINT64_MAX);
   MOZ_ASSERT(mLastModificationDate == INT64_MAX);
 
   uint64_t totalLength = 0;
   int64_t lastModified = 0;
   bool lastModifiedSet = false;
 
   for (uint32_t index = 0, count = mBlobImpls.Length(); index < count; index++) {
     RefPtr<BlobImpl>& blob = mBlobImpls[index];
 
 #ifdef DEBUG
     MOZ_ASSERT(!blob->IsSizeUnknown());
     MOZ_ASSERT(!blob->IsDateUnknown());
 #endif
 
-    uint64_t subBlobLength = blob->GetSize(aRv);
-    if (NS_WARN_IF(aRv.Failed())) {
-      return;
-    }
+    ErrorResult error;
+    uint64_t subBlobLength = blob->GetSize(error);
+    MOZ_ALWAYS_TRUE(!error.Failed());
 
     MOZ_ASSERT(UINT64_MAX - subBlobLength >= totalLength);
     totalLength += subBlobLength;
 
     if (blob->IsFile()) {
-      int64_t partLastModified = blob->GetLastModified(aRv);
-      if (NS_WARN_IF(aRv.Failed())) {
-        return;
-      }
+      int64_t partLastModified = blob->GetLastModified(error);
+      MOZ_ALWAYS_TRUE(!error.Failed());
 
       if (lastModified < partLastModified) {
         lastModified = partLastModified;
         lastModifiedSet = true;
       }
     }
   }
 
@@ -348,18 +309,17 @@ MultipartBlobImpl::InitializeChromeFile(
     aBlob.GetType(mContentType);
   }
 
 
   BlobSet blobSet;
   blobSet.AppendBlobImpl(aBlob.Impl());
   mBlobImpls = blobSet.GetBlobImpls();
 
-  SetLengthAndModifiedDate(aRv);
-  NS_WARN_IF(aRv.Failed());
+  SetLengthAndModifiedDate();
 }
 
 void
 MultipartBlobImpl::InitializeChromeFile(nsPIDOMWindow* aWindow,
                                         nsIFile* aFile,
                                         const ChromeFilePropertyBag& aBag,
                                         bool aIsFromNsIFile,
                                         ErrorResult& aRv)
@@ -420,18 +380,17 @@ MultipartBlobImpl::InitializeChromeFile(
   if (mContentType.IsEmpty()) {
     blob->GetType(mContentType);
   }
 
   BlobSet blobSet;
   blobSet.AppendBlobImpl(static_cast<File*>(blob.get())->Impl());
   mBlobImpls = blobSet.GetBlobImpls();
 
-  SetLengthAndModifiedDate(aRv);
-  NS_WARN_IF(aRv.Failed());
+  SetLengthAndModifiedDate();
 }
 
 void
 MultipartBlobImpl::InitializeChromeFile(nsPIDOMWindow* aWindow,
                                         const nsAString& aData,
                                         const ChromeFilePropertyBag& aBag,
                                         ErrorResult& aRv)
 {

--- a/dom/base/MultipartBlobImpl.h
+++ b/dom/base/MultipartBlobImpl.h
@@ -20,43 +20,51 @@ using namespace mozilla;
 using namespace mozilla::dom;
 
 class MultipartBlobImpl final : public BlobImplBase
 {
 public:
   NS_DECL_ISUPPORTS_INHERITED
 
   // Create as a file
-  static already_AddRefed<MultipartBlobImpl>
-  Create(const nsTArray<RefPtr<BlobImpl>>& aBlobImpls,
-         const nsAString& aName,
-         const nsAString& aContentType,
-         ErrorResult& aRv);
+  MultipartBlobImpl(const nsTArray<RefPtr<BlobImpl>>& aBlobImpls,
+                    const nsAString& aName,
+                    const nsAString& aContentType)
+    : BlobImplBase(aName, aContentType, UINT64_MAX),
+      mBlobImpls(aBlobImpls),
+      mIsFromNsIFile(false)
+  {
+    SetLengthAndModifiedDate();
+  }
 
   // Create as a blob
-  static already_AddRefed<MultipartBlobImpl>
-  Create(const nsTArray<RefPtr<BlobImpl>>& aBlobImpls,
-         const nsAString& aContentType,
-         ErrorResult& aRv);
+  MultipartBlobImpl(const nsTArray<RefPtr<BlobImpl>>& aBlobImpls,
+                    const nsAString& aContentType)
+    : BlobImplBase(aContentType, UINT64_MAX),
+      mBlobImpls(aBlobImpls),
+      mIsFromNsIFile(false)
+  {
+    SetLengthAndModifiedDate();
+  }
 
   // Create as a file to be later initialized
   explicit MultipartBlobImpl(const nsAString& aName)
     : BlobImplBase(aName, EmptyString(), UINT64_MAX),
       mIsFromNsIFile(false)
   {
   }
 
   // Create as a blob to be later initialized
   MultipartBlobImpl()
     : BlobImplBase(EmptyString(), UINT64_MAX),
       mIsFromNsIFile(false)
   {
   }
 
-  void InitializeBlob(ErrorResult& aRv);
+  void InitializeBlob();
 
   void InitializeBlob(
        JSContext* aCx,
        const Sequence<OwningArrayBufferOrArrayBufferViewOrBlobOrString>& aData,
        const nsAString& aContentType,
        bool aNativeEOL,
        ErrorResult& aRv);
 
@@ -107,34 +115,17 @@ public:
   void SetFromNsIFile(bool aValue)
   {
     mIsFromNsIFile = aValue;
   }
 
   virtual bool MayBeClonedToOtherThreads() const override;
 
 protected:
-  MultipartBlobImpl(const nsTArray<RefPtr<BlobImpl>>& aBlobImpls,
-                    const nsAString& aName,
-                    const nsAString& aContentType)
-    : BlobImplBase(aName, aContentType, UINT64_MAX),
-      mBlobImpls(aBlobImpls),
-      mIsFromNsIFile(false)
-  {
-  }
-
-  MultipartBlobImpl(const nsTArray<RefPtr<BlobImpl>>& aBlobImpls,
-                    const nsAString& aContentType)
-    : BlobImplBase(aContentType, UINT64_MAX),
-      mBlobImpls(aBlobImpls),
-      mIsFromNsIFile(false)
-  {
-  }
-
   virtual ~MultipartBlobImpl() {}
 
-  void SetLengthAndModifiedDate(ErrorResult& aRv);
+  void SetLengthAndModifiedDate();
 
   nsTArray<RefPtr<BlobImpl>> mBlobImpls;
   bool mIsFromNsIFile;
 };
 
 #endif // mozilla_dom_MultipartBlobImpl_h

--- a/dom/base/StructuredCloneHolder.cpp
+++ b/dom/base/StructuredCloneHolder.cpp
@@ -550,18 +550,17 @@ StructuredCloneHolder::WriteFullySeriali
   return false;
 }
 
 namespace {
 
 // Recursive!
 already_AddRefed<BlobImpl>
 EnsureBlobForBackgroundManager(BlobImpl* aBlobImpl,
-                               PBackgroundChild* aManager,
-                               ErrorResult& aRv)
+                               PBackgroundChild* aManager = nullptr)
 {
   MOZ_ASSERT(aBlobImpl);
   RefPtr<BlobImpl> blobImpl = aBlobImpl;
 
   if (!aManager) {
     aManager = BackgroundChild::GetForCurrentThread();
     if (!aManager) {
       return blobImpl.forget();
@@ -600,44 +599,35 @@ EnsureBlobForBackgroundManager(BlobImpl*
   bool newBlobImplNeeded = false;
 
   for (uint32_t index = 0; index < subBlobCount; index++) {
     const RefPtr<BlobImpl>& subBlobImpl = subBlobImpls->ElementAt(index);
     MOZ_ASSERT(subBlobImpl);
 
     RefPtr<BlobImpl>& newSubBlobImpl = newSubBlobImpls[index];
 
-    newSubBlobImpl = EnsureBlobForBackgroundManager(subBlobImpl, aManager, aRv);
-    if (NS_WARN_IF(aRv.Failed())) {
-      return nullptr;
-    }
-
+    newSubBlobImpl = EnsureBlobForBackgroundManager(subBlobImpl, aManager);
     MOZ_ASSERT(newSubBlobImpl);
 
     if (subBlobImpl != newSubBlobImpl) {
       newBlobImplNeeded = true;
     }
   }
 
   if (newBlobImplNeeded) {
     nsString contentType;
     blobImpl->GetType(contentType);
 
     if (blobImpl->IsFile()) {
       nsString name;
       blobImpl->GetName(name);
 
-      blobImpl = MultipartBlobImpl::Create(newSubBlobImpls, name,
-                                           contentType, aRv);
+      blobImpl = new MultipartBlobImpl(newSubBlobImpls, name, contentType);
     } else {
-      blobImpl = MultipartBlobImpl::Create(newSubBlobImpls, contentType, aRv);
-    }
-
-    if (NS_WARN_IF(aRv.Failed())) {
-      return nullptr;
+      blobImpl = new MultipartBlobImpl(newSubBlobImpls, contentType);
     }
 
     MOZ_ALWAYS_TRUE(NS_SUCCEEDED(blobImpl->SetMutable(false)));
   }
 
   return blobImpl.forget();
 }
 
@@ -645,23 +635,17 @@ JSObject*
 ReadBlob(JSContext* aCx,
          uint32_t aIndex,
          StructuredCloneHolder* aHolder)
 {
   MOZ_ASSERT(aHolder);
   MOZ_ASSERT(aIndex < aHolder->BlobImpls().Length());
   RefPtr<BlobImpl> blobImpl = aHolder->BlobImpls()[aIndex];
 
-  ErrorResult rv;
-  blobImpl = EnsureBlobForBackgroundManager(blobImpl, nullptr, rv);
-  if (NS_WARN_IF(rv.Failed())) {
-    rv.SuppressException();
-    return nullptr;
-  }
-
+  blobImpl = EnsureBlobForBackgroundManager(blobImpl);
   MOZ_ASSERT(blobImpl);
 
   // RefPtr<File> needs to go out of scope before toObjectOrNull() is
   // called because the static analysis thinks dereferencing XPCOM objects
   // can GC (because in some cases it can!), and a return statement with a
   // JSObject* type means that JSObject* is on the stack as a raw pointer
   // while destructors are running.
   JS::Rooted<JS::Value> val(aCx);
@@ -679,24 +663,17 @@ bool
 WriteBlob(JSStructuredCloneWriter* aWriter,
           Blob* aBlob,
           StructuredCloneHolder* aHolder)
 {
   MOZ_ASSERT(aWriter);
   MOZ_ASSERT(aBlob);
   MOZ_ASSERT(aHolder);
 
-  ErrorResult rv;
-  RefPtr<BlobImpl> blobImpl =
-    EnsureBlobForBackgroundManager(aBlob->Impl(), nullptr, rv);
-  if (NS_WARN_IF(rv.Failed())) {
-    rv.SuppressException();
-    return false;
-  }
-
+  RefPtr<BlobImpl> blobImpl = EnsureBlobForBackgroundManager(aBlob->Impl());
   MOZ_ASSERT(blobImpl);
 
   MOZ_ALWAYS_TRUE(NS_SUCCEEDED(blobImpl->SetMutable(false)));
 
   // We store the position of the blobImpl in the array as index.
   if (JS_WriteUint32Pair(aWriter, SCTAG_DOM_BLOB,
                          aHolder->BlobImpls().Length())) {
     aHolder->BlobImpls().AppendElement(blobImpl);
@@ -732,23 +709,17 @@ ReadFileList(JSContext* aCx,
     // |aCount| is the number of BlobImpls to use from the |offset|.
     for (uint32_t i = 0; i < aCount; ++i) {
       uint32_t index = offset + i;
       MOZ_ASSERT(index < aHolder->BlobImpls().Length());
 
       RefPtr<BlobImpl> blobImpl = aHolder->BlobImpls()[index];
       MOZ_ASSERT(blobImpl->IsFile());
 
-      ErrorResult rv;
-      blobImpl = EnsureBlobForBackgroundManager(blobImpl, nullptr, rv);
-      if (NS_WARN_IF(rv.Failed())) {
-        rv.SuppressException();
-        return nullptr;
-      }
-
+      blobImpl = EnsureBlobForBackgroundManager(blobImpl);
       MOZ_ASSERT(blobImpl);
 
       RefPtr<File> file = File::Create(aHolder->ParentDuringRead(), blobImpl);
       if (!fileList->Append(file)) {
         return nullptr;
       }
     }
 
@@ -777,32 +748,24 @@ WriteFileList(JSStructuredCloneWriter* a
   // starting from the offset.
   if (!JS_WriteUint32Pair(aWriter, SCTAG_DOM_FILELIST,
                           aFileList->Length()) ||
       !JS_WriteUint32Pair(aWriter, 0,
                           aHolder->BlobImpls().Length())) {
     return false;
   }
 
-  ErrorResult rv;
-  nsTArray<RefPtr<BlobImpl>> blobImpls;
-
   for (uint32_t i = 0; i < aFileList->Length(); ++i) {
     RefPtr<BlobImpl> blobImpl =
-      EnsureBlobForBackgroundManager(aFileList->Item(i)->Impl(), nullptr, rv);
-    if (NS_WARN_IF(rv.Failed())) {
-      rv.SuppressException();
-      return false;
-    }
+      EnsureBlobForBackgroundManager(aFileList->Item(i)->Impl());
+    MOZ_ASSERT(blobImpl);
 
-    MOZ_ASSERT(blobImpl);
-    blobImpls.AppendElement(blobImpl);
+    aHolder->BlobImpls().AppendElement(blobImpl);
   }
 
-  aHolder->BlobImpls().AppendElements(blobImpls);
   return true;
 }
 
 // Read the WriteFormData for the format.
 JSObject*
 ReadFormData(JSContext* aCx,
              JSStructuredCloneReader* aReader,
              uint32_t aCount,
@@ -836,38 +799,29 @@ ReadFormData(JSContext* aCx,
         RefPtr<BlobImpl> blobImpl =
           aHolder->BlobImpls()[indexOrLengthOfString];
         MOZ_ASSERT(blobImpl->IsFile());
 
         RefPtr<File> file =
           File::Create(aHolder->ParentDuringRead(), blobImpl);
         MOZ_ASSERT(file);
 
-        ErrorResult rv;
-        formData->Append(name, *file, thirdArg, rv);
-        if (NS_WARN_IF(rv.Failed())) {
-          return nullptr;
-        }
-
+        formData->Append(name, *file, thirdArg);
       } else {
         MOZ_ASSERT(tag == 0);
 
         nsAutoString value;
         value.SetLength(indexOrLengthOfString);
         size_t charSize = sizeof(nsString::char_type);
         if (!JS_ReadBytes(aReader, (void*) value.BeginWriting(),
                           indexOrLengthOfString * charSize)) {
           return nullptr;
         }
 
-        ErrorResult rv;
-        formData->Append(name, value, rv);
-        if (NS_WARN_IF(rv.Failed())) {
-          return nullptr;
-        }
+        formData->Append(name, value);
       }
     }
 
     if (!ToJSValue(aCx, formData, &val)) {
       return nullptr;
     }
   }
 

--- a/dom/base/nsFormData.cpp
+++ b/dom/base/nsFormData.cpp
@@ -20,18 +20,17 @@ nsFormData::nsFormData(nsISupports* aOwn
   , mOwner(aOwner)
 {
 }
 
 namespace {
 
 // Implements steps 3 and 4 of the "create an entry" algorithm of FormData.
 already_AddRefed<File>
-CreateNewFileInstance(Blob& aBlob, const Optional<nsAString>& aFilename,
-                      ErrorResult& aRv)
+CreateNewFileInstance(Blob& aBlob, const Optional<nsAString>& aFilename)
 {
   // Step 3 "If value is a Blob object and not a File object, set value to
   // a new File object, representing the same bytes, whose name attribute value
   // is "blob"."
   // Step 4 "If value is a File object and filename is given, set value to
   // a new File object, representing the same bytes, whose name attribute
   // value is filename."
   nsAutoString filename;
@@ -43,22 +42,17 @@ CreateNewFileInstance(Blob& aBlob, const
     RefPtr<File> file = aBlob.ToFile();
     if (file) {
       return file.forget();
     }
 
     filename = NS_LITERAL_STRING("blob");
   }
 
-  RefPtr<File> file = aBlob.ToFile(filename, aRv);
-  if (NS_WARN_IF(aRv.Failed())) {
-    return nullptr;
-  }
-
-  return file.forget();
+  return aBlob.ToFile(filename);
 }
 
 } // namespace
 
 // -------------------------------------------------------------------------
 // nsISupports
 
 NS_IMPL_CYCLE_COLLECTION_CLASS(nsFormData)
@@ -102,32 +96,26 @@ nsresult
 nsFormData::GetEncodedSubmission(nsIURI* aURI,
                                  nsIInputStream** aPostDataStream)
 {
   NS_NOTREACHED("Shouldn't call nsFormData::GetEncodedSubmission");
   return NS_OK;
 }
 
 void
-nsFormData::Append(const nsAString& aName, const nsAString& aValue,
-                   ErrorResult& aRv)
+nsFormData::Append(const nsAString& aName, const nsAString& aValue)
 {
   AddNameValuePair(aName, aValue);
 }
 
 void
 nsFormData::Append(const nsAString& aName, Blob& aBlob,
-                   const Optional<nsAString>& aFilename,
-                   ErrorResult& aRv)
+                   const Optional<nsAString>& aFilename)
 {
-  RefPtr<File> file = CreateNewFileInstance(aBlob, aFilename, aRv);
-  if (NS_WARN_IF(aRv.Failed())) {
-    return;
-  }
-
+  RefPtr<File> file = CreateNewFileInstance(aBlob, aFilename);
   AddNameFilePair(aName, file);
 }
 
 void
 nsFormData::Delete(const nsAString& aName)
 {
   // We have to use this slightly awkward for loop since uint32_t >= 0 is an
   // error for being always true.
@@ -203,41 +191,35 @@ nsFormData::RemoveAllOthersAndGetFirstFo
     }
   }
 
   return lastFoundTuple;
 }
 
 void
 nsFormData::Set(const nsAString& aName, Blob& aBlob,
-                const Optional<nsAString>& aFilename,
-                ErrorResult& aRv)
+                const Optional<nsAString>& aFilename)
 {
   FormDataTuple* tuple = RemoveAllOthersAndGetFirstFormDataTuple(aName);
   if (tuple) {
-    RefPtr<File> file = CreateNewFileInstance(aBlob, aFilename, aRv);
-    if (NS_WARN_IF(aRv.Failed())) {
-      return;
-    }
-
+    RefPtr<File> file = CreateNewFileInstance(aBlob, aFilename);
     SetNameFilePair(tuple, aName, file);
   } else {
-    Append(aName, aBlob, aFilename, aRv);
+    Append(aName, aBlob, aFilename);
   }
 }
 
 void
-nsFormData::Set(const nsAString& aName, const nsAString& aValue,
-                ErrorResult& aRv)
+nsFormData::Set(const nsAString& aName, const nsAString& aValue)
 {
   FormDataTuple* tuple = RemoveAllOthersAndGetFirstFormDataTuple(aName);
   if (tuple) {
     SetNameValuePair(tuple, aName, aValue);
   } else {
-    Append(aName, aValue, aRv);
+    Append(aName, aValue);
   }
 }
 
 uint32_t
 nsFormData::GetIterableLength() const
 {
   return mFormData.Length();
 }
@@ -274,40 +256,30 @@ nsFormData::Append(const nsAString& aNam
     NS_ENSURE_SUCCESS(rv, rv);
 
     free(iid);
 
     nsCOMPtr<nsIDOMBlob> domBlob = do_QueryInterface(supports);
     RefPtr<Blob> blob = static_cast<Blob*>(domBlob.get());
     if (domBlob) {
       Optional<nsAString> temp;
-      ErrorResult rv;
-      Append(aName, *blob, temp, rv);
-      if (NS_WARN_IF(rv.Failed())) {
-        return rv.StealNSResult();
-      }
-
+      Append(aName, *blob, temp);
       return NS_OK;
     }
   }
 
   char16_t* stringData = nullptr;
   uint32_t stringLen = 0;
   rv = aValue->GetAsWStringWithSize(&stringLen, &stringData);
   NS_ENSURE_SUCCESS(rv, rv);
 
   nsString valAsString;
   valAsString.Adopt(stringData, stringLen);
 
-  ErrorResult error;
-  Append(aName, valAsString, error);
-  if (NS_WARN_IF(error.Failed())) {
-    return error.StealNSResult();
-  }
-
+  Append(aName, valAsString);
   return NS_OK;
 }
 
 /* virtual */ JSObject*
 nsFormData::WrapObject(JSContext* aCx, JS::Handle<JSObject*> aGivenProto)
 {
   return FormDataBinding::Wrap(aCx, this, aGivenProto);
 }

--- a/dom/base/nsFormData.h
+++ b/dom/base/nsFormData.h
@@ -88,30 +88,26 @@ public:
   GetParentObject() const
   {
     return mOwner;
   }
   static already_AddRefed<nsFormData>
   Constructor(const mozilla::dom::GlobalObject& aGlobal,
               const mozilla::dom::Optional<mozilla::dom::NonNull<mozilla::dom::HTMLFormElement> >& aFormElement,
               mozilla::ErrorResult& aRv);
-  void Append(const nsAString& aName, const nsAString& aValue,
-              mozilla::ErrorResult& aRv);
+  void Append(const nsAString& aName, const nsAString& aValue);
   void Append(const nsAString& aName, Blob& aBlob,
-              const mozilla::dom::Optional<nsAString>& aFilename,
-              mozilla::ErrorResult& aRv);
+              const mozilla::dom::Optional<nsAString>& aFilename);
   void Delete(const nsAString& aName);
   void Get(const nsAString& aName, mozilla::dom::Nullable<OwningFileOrUSVString>& aOutValue);
   void GetAll(const nsAString& aName, nsTArray<OwningFileOrUSVString>& aValues);
   bool Has(const nsAString& aName);
   void Set(const nsAString& aName, Blob& aBlob,
-           const mozilla::dom::Optional<nsAString>& aFilename,
-           mozilla::ErrorResult& aRv);
-  void Set(const nsAString& aName, const nsAString& aValue,
-           mozilla::ErrorResult& aRv);
+           const mozilla::dom::Optional<nsAString>& aFilename);
+  void Set(const nsAString& aName, const nsAString& aValue);
 
   uint32_t GetIterableLength() const;
   const nsAString& GetKeyAtIndex(uint32_t aIndex) const;
   const OwningFileOrUSVString& GetValueAtIndex(uint32_t aIndex) const;
 
   // nsFormSubmission
   virtual nsresult GetEncodedSubmission(nsIURI* aURI,
                                         nsIInputStream** aPostDataStream) override;

--- a/dom/base/nsXMLHttpRequest.cpp
+++ b/dom/base/nsXMLHttpRequest.cpp
@@ -793,42 +793,43 @@ nsXMLHttpRequest::CreateResponseParsedJS
     return NS_ERROR_FAILURE;
   }
 
   mResultJSON = value;
   return NS_OK;
 }
 
 void
-nsXMLHttpRequest::CreatePartialBlob(ErrorResult& aRv)
+nsXMLHttpRequest::CreatePartialBlob()
 {
   if (mDOMBlob) {
     // Use progress info to determine whether load is complete, but use
     // mDataAvailable to ensure a slice is created based on the uncompressed
     // data count.
     if (mLoadTotal == mLoadTransferred) {
       mResponseBlob = mDOMBlob;
     } else {
+      ErrorResult rv;
       mResponseBlob = mDOMBlob->CreateSlice(0, mDataAvailable,
-                                            EmptyString(), aRv);
+                                            EmptyString(), rv);
     }
     return;
   }
 
   // mBlobSet can be null if the request has been canceled
   if (!mBlobSet) {
     return;
   }
 
   nsAutoCString contentType;
   if (mLoadTotal == mLoadTransferred) {
     mChannel->GetContentType(contentType);
   }
 
-  mResponseBlob = mBlobSet->GetBlobInternal(GetOwner(), contentType, aRv);
+  mResponseBlob = mBlobSet->GetBlobInternal(GetOwner(), contentType);
 }
 
 NS_IMETHODIMP nsXMLHttpRequest::GetResponseType(nsAString& aResponseType)
 {
   switch (mResponseType) {
   case XML_HTTP_RESPONSE_TYPE_DEFAULT:
     aResponseType.Truncate();
     break;
@@ -1012,17 +1013,17 @@ nsXMLHttpRequest::GetResponse(JSContext*
   {
     if (!(mState & XML_HTTP_REQUEST_DONE)) {
       if (mResponseType != XML_HTTP_RESPONSE_TYPE_MOZ_BLOB) {
         aResponse.setNull();
         return;
       }
 
       if (!mResponseBlob) {
-        CreatePartialBlob(aRv);
+        CreatePartialBlob();
       }
     }
 
     if (!mResponseBlob) {
       aResponse.setNull();
       return;
     }
 
@@ -2205,24 +2206,18 @@ nsXMLHttpRequest::OnStopRequest(nsIReque
       // and if the response length is zero.
       if (!mBlobSet) {
         mBlobSet = new BlobSet();
       }
       // Smaller files may be written in cache map instead of separate files.
       // Also, no-store response cannot be written in persistent cache.
       nsAutoCString contentType;
       mChannel->GetContentType(contentType);
-
-      ErrorResult rv;
-      mResponseBlob = mBlobSet->GetBlobInternal(GetOwner(), contentType, rv);
+      mResponseBlob = mBlobSet->GetBlobInternal(GetOwner(), contentType);
       mBlobSet = nullptr;
-
-      if (NS_WARN_IF(rv.Failed())) {
-        return rv.StealNSResult();
-      }
     }
     NS_ASSERTION(mResponseBody.IsEmpty(), "mResponseBody should be empty");
     NS_ASSERTION(mResponseText.IsEmpty(), "mResponseText should be empty");
   } else if (NS_SUCCEEDED(status) &&
              ((mResponseType == XML_HTTP_RESPONSE_TYPE_ARRAYBUFFER &&
                !mIsMappedArrayBuffer) ||
               mResponseType == XML_HTTP_RESPONSE_TYPE_CHUNKED_ARRAYBUFFER)) {
     // set the capacity down to the actual length, to realloc back

--- a/dom/base/nsXMLHttpRequest.h
+++ b/dom/base/nsXMLHttpRequest.h
@@ -600,17 +600,17 @@ protected:
   nsresult AppendToResponseText(const char * aBuffer, uint32_t aBufferLen);
   static NS_METHOD StreamReaderFunc(nsIInputStream* in,
                 void* closure,
                 const char* fromRawSegment,
                 uint32_t toOffset,
                 uint32_t count,
                 uint32_t *writeCount);
   nsresult CreateResponseParsedJSON(JSContext* aCx);
-  void CreatePartialBlob(ErrorResult& aRv);
+  void CreatePartialBlob();
   bool CreateDOMBlob(nsIRequest *request);
   // Change the state of the object with this. The broadcast argument
   // determines if the onreadystatechange listener should be called.
   nsresult ChangeState(uint32_t aState, bool aBroadcast = true);
   already_AddRefed<nsILoadGroup> GetLoadGroup() const;
   nsIURI *GetBaseURI();
 
   already_AddRefed<nsIHttpChannel> GetCurrentHttpChannel();

--- a/dom/fetch/FetchUtil.cpp
+++ b/dom/fetch/FetchUtil.cpp
@@ -198,19 +198,17 @@ public:
     : mFormData(aFormData)
   {
     MOZ_ASSERT(aFormData);
   }
 
   bool URLParamsIterator(const nsString& aName,
                          const nsString& aValue) override
   {
-    ErrorResult rv;
-    mFormData->Append(aName, aValue, rv);
-    MOZ_ASSERT(!rv.Failed());
+    mFormData->Append(aName, aValue);
     return true;
   }
 
 private:
   nsFormData* mFormData;
 };
 
 /**
@@ -389,19 +387,17 @@ private:
 
     if (!mFormData) {
       mFormData = new nsFormData();
     }
 
     NS_ConvertUTF8toUTF16 name(mName);
 
     if (mFilename.IsVoid()) {
-      ErrorResult rv;
-      mFormData->Append(name, NS_ConvertUTF8toUTF16(body), rv);
-      MOZ_ASSERT(!rv.Failed());
+      mFormData->Append(name, NS_ConvertUTF8toUTF16(body));
     } else {
       // Unfortunately we've to copy the data first since all our strings are
       // going to free it. We also need fallible alloc, so we can't just use
       // ToNewCString().
       char* copy = static_cast<char*>(moz_xmalloc(body.Length()));
       if (!copy) {
         NS_WARNING("Failed to copy File entry body.");
         return false;
@@ -416,21 +412,17 @@ private:
       p = nullptr;
 
       RefPtr<Blob> file =
         File::CreateMemoryFile(mParentObject,
                                reinterpret_cast<void *>(copy), body.Length(),
                                NS_ConvertUTF8toUTF16(mFilename),
                                NS_ConvertUTF8toUTF16(mContentType), /* aLastModifiedDate */ 0);
       Optional<nsAString> dummy;
-      ErrorResult rv;
-      mFormData->Append(name, *file, dummy, rv);
-      if (NS_WARN_IF(rv.Failed())) {
-        return false;
-      }
+      mFormData->Append(name, *file, dummy);
     }
 
     return true;
   }
 
 public:
   FormDataParser(const nsACString& aMimeType, const nsACString& aData, nsIGlobalObject* aParent)
     : mMimeType(aMimeType), mData(aData), mState(START_PART), mParentObject(aParent)

--- a/dom/ipc/Blob.cpp
+++ b/dom/ipc/Blob.cpp
@@ -899,27 +899,23 @@ CreateBlobImpl(const nsTArray<BlobData>&
       return nullptr;
     }
 
     DebugOnly<bool> isMutable;
     MOZ_ASSERT(NS_SUCCEEDED(blobImpl->GetMutable(&isMutable)));
     MOZ_ASSERT(!isMutable);
   }
 
-  ErrorResult rv;
   RefPtr<BlobImpl> blobImpl;
   if (!hasRecursed && aMetadata.IsFile()) {
-    blobImpl = MultipartBlobImpl::Create(blobImpls, aMetadata.mName,
-                                         aMetadata.mContentType, rv);
+    blobImpl =
+      new MultipartBlobImpl(blobImpls, aMetadata.mName, aMetadata.mContentType);
   } else {
-    blobImpl = MultipartBlobImpl::Create(blobImpls, aMetadata.mContentType, rv);
-  }
-
-  if (NS_WARN_IF(rv.Failed())) {
-    return nullptr;
+    blobImpl =
+      new MultipartBlobImpl(blobImpls, aMetadata.mContentType);
   }
 
   MOZ_ALWAYS_TRUE(NS_SUCCEEDED(blobImpl->SetMutable(false)));
 
   return blobImpl.forget();
 }
 
 already_AddRefed<BlobImpl>

--- a/dom/webidl/FormData.webidl
+++ b/dom/webidl/FormData.webidl
@@ -7,22 +7,18 @@
  * http://xhr.spec.whatwg.org
  */
 
 typedef (File or USVString) FormDataEntryValue;
 
 [Constructor(optional HTMLFormElement form),
  Exposed=(Window,Worker)]
 interface FormData {
-  [Throws]
   void append(USVString name, Blob value, optional USVString filename);
-  [Throws]
   void append(USVString name, USVString value);
   void delete(USVString name);
   FormDataEntryValue? get(USVString name);
   sequence<FormDataEntryValue> getAll(USVString name);
   boolean has(USVString name);
-  [Throws]
   void set(USVString name, Blob value, optional USVString filename);
-  [Throws]
   void set(USVString name, USVString value);
   iterable<USVString, FormDataEntryValue>;
 };